My chemistry textbook:
“A catalyst is a substance that increases the rate of reaction without being consumed in the overall reaction.”
The Complete Idiot’s Guide to Chemistry:
“Catalysts can be compared to my friend Erika (not that you know her). When I was in college, Erika was roommates with a woman named Ingrid. Erika helped to set us up, and I ended up marrying Ingrid. Though Erika was involved in the process (by introducing me to her roommate), she didn’t actually take part in the resulting reaction. Therefore she was a catalyst for that process.”